The present invention relates generally to highly ruggedized electro-optical devices for detecting radiation within a harsh environment. More particularly, the present invention relates to a radiation detecting device with a protective shield of standard size and including a scintillation element whose volume has been optimized.
The present invention further relates to a radiation detecting device utilizing numerous elements in combination to provide an effective dynamic suspension system to reduce noise.
Radiation detectors are well known in the drilling industry and are often incorporated into drilling tools for oil wells and the like. Radiation detectors typically include a light detecting and quantifying device, such as a photomultiplier tube, and a scintillation crystal element, or a suitably compounded element. The scintillation element functions by capturing radiation from the formation around a well bore. The radiation may be ambient radiation emitted by radioactive materials in the formation, or radiation emitted in response to bombardment of the formation by radiation sources within the drilling or logging equipment.
A scintillation element responds to gamma radiation by transforming the radiation energy into light energy. The light energy is transmitted through an optical window into a light detecting device, such as a photomultiplier tube. The light impulses are transformed into electrical impulses which are transmitted via a data stream to a computerized instrumentation system.
A radiation detector may be incorporated into a variety of instrumentation and/or control systems where harsh environments exist. The process of well logging the formations in which oil wells are drilled utilizes radiation detectors that are lowered via wireline into the well bores. As the instrument string is rapidly moved through the bore, the detectors are subjected to considerable shock and to temperatures as high as about 200.degree. C. Measurement While Drilling (MWD) or Logging While Drilling (LWD) operations utilize radiation detectors to help evaluate the formations concurrently with the drilling operations and may be used to help guide the drill, thereby subjecting the radiation detector to extreme levels of shock and vibration while also being subjected to temperatures as high as about 175.degree. C. or higher in exceptional cases. Other types of drilling operations, such as for environmental evaluations, geologic evaluations, and support to construction often require similar uses of radiation detectors.
Radiation detectors may also be used in coal mines to detect the boundary between the coal and the formations above and below the coal. Positioning of the detectors on mining equipment, such as continuous miners, near the point where the coal is being removed from the formation subjects the detectors to extreme levels of shock and vibration and above ambient temperatures.
In all the above-noted instances, a highly ruggedized detector is essential so that the detector will not fail and will not produce noise as a result of the shock and vibration.
A major obstacle faced in the design of radiation detector packages relates to the space available to incorporate all the necessary components. Very little space is available in well bores for the placement of radiation detectors, dictating that the detectors be as small as possible. On continuous miners, small detectors are desirable so that they can be strategically located on the machinery for best performance. The need to minimize the size of detector packages is made greater by a trend in the oil drilling industry to utilize smaller diameter drills in cutting well bores. This leads to the need to maximize the use of the space available for radiation detectors.
While it is necessary to have radiation detectors which are small in size, it is imperative that scintillation elements themselves remain as large as possible for at least two reasons. First, a larger cross-section increases the probability that a gamma ray will pass into the scintillation element. Second, a greater thickness of crystal material increases the probability that gamma rays will produce scintillation, rather than just pass through the scintillation element.
Prior art attempts at providing a radiation detector with an efficient scintillation element have not focused on maximizing the size of the element. U.S. Pat. No. 5,241,180 (Ishaque et al.) refers to radiation detectors having a scintillation element with a large end and a smaller end. The large end is nearest the source of radiation, and the element tapers down to the small end positioned near the photomultiplier tube. However, this tapered configuration does not have an optimized volume. Further, this configuration requires a specially constructed detector housing in order to properly house and protect the element from shock.
U.S. Pat. No. 5,614,721 (Pandelisev) refers to a modular gamma camera plate which utilizes scintillation elements of various configurations. However, none of the configurations referred to have an optimized volume. Further, like Ishaque et al., the element of Pandelisev must be housed in a specially constructed housing.
While it is important to maximize the size of a scintillation element, it is necessary to provide a design which effectively protects the scintillation element in a harsh environment. A stiff support system is necessary to maintain a high resonant frequency so that any shock and/or vibration induced displacement of the element will be small. Motion of the element relative to the materials around the element must be kept very small in order to avoid false scintillations which appear as random noise in the data stream and degrade the performance of the detector. In many earlier designs, this noise has been excessive so that the detectors were found to be useless for these extreme environments. This is particularly true when the crystal moves enough to decouple from the window. In addition to providing a high resonant frequency, there must be enough restraining force to prevent the crystal from decoupling during high shock in the axial direction. As the crystal size is made larger within a given sized envelope, less space is available for the support system around the crystal, making it more difficult to incorporate the desired dynamic stiffness and restraining forces. The present invention provides a significant increase in volume of the scintillation element while, at the same time, provides a dynamic support system superior to conventional designs.
Another major complication to designing detectors for the environments described above is the effect of high temperatures. Providing a stiff dynamic support system can produce destructive forces when the scintillation element expands with increasing temperature. This is particularly true when the scintillation element has a high coefficient of thermal expansion, as in the case when the element is made from sodium iodide. Sodium iodide has a coefficient of thermal expansion that is high compared with the metals required for a hermetically sealed shield around the element. Similarly, a high restraining force placed on the crystal in order to keep the crystal from decoupling from the window may combine with other forces that will result in breaking the crystal, breaking the window, or degrading the support elements, which then results in noise or breakage. The present invention provides more effective compliance with thermal expansion while simultaneously providing larger volume, high dynamic stiffness and high restraining forces.
Prior art attempts at providing a radiation detector which operates with minimal noise have been unsuccessful. U.S. Pat. No. 4,900,937 (Dayton et al.) utilizes a weak biasing spring at the non-window end of a scintillation crystal. The spring exerts a force no greater than one hundred fifty times the weight of the scintillation crystal and provides no other significant restraining force in the axial direction. Due to this weak biasing force, the crystal is allowed to decouple from the window in response to shock forces on the detector in the axial direction. Such decoupling causes noise. The noise is introduced into the data stream being transmitted from the photomultiplier tube to the instrumentation system, degrading the performance of the system. A design which allows for decoupling of the scintillation element from the window requires that the radial forces by the sidewalls be weak, such that the element will be able to move back from the window and optical coupler under high shock. Such a radial support may not be stiff enough to provide the high resonant frequency required to sufficiently constrain motion in the radial direction to prevent noise.
U.S. Pat. Nos. 4,004,151 (Novak), 4,360,733 (Novak et al.), 4,383,175 (Toepke) and 4,764,677 (Spurney) all utilize a biasing means at the non-window end of the crystal which provides a strong biasing force to prevent decoupling of the crystal from the window.
Problems arise from the use of a strong biasing means directed toward the window from a single location, namely the non-window end of the crystal. This is particularly true if the window is made from glass or materials that are no stronger than glass. If such high forces are provided by a stiff spring alone, the forces at high temperatures, particularly when combined with high shock, may break the scintillation element or the window. If a softer spring is used but is highly compressed in order to produce the high biasing force, it will not provide the high stiffness required to maintain a high resonant frequency.
In addition, the magnitude of the biasing force is of lesser importance than the overall effective dynamic stiffness of the detector package. The prior art has not provided a scintillation detector package which has an effective dynamic stiffness capable of creating a high resonant frequency, and thus minimized noise, for the package.